ACPI / video: Always call acpi_video_init_brightness() on init

We have to call acpi_video_init_brightness() even if we're not going
to initialise the backlight - Thinkpads seem to use this as the
trigger for enabling ACPI notifications rather than handling it in
firmware.

[rjw: Drop the brightness object created by
 acpi_video_init_brightness() if we are not going to use it.]
Signed-off-by: Matthew Garrett <matthew.garrett@nebula.com>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>